1. Field of the Invention
The present invention relates to a magnetic element making use of a tunnel current, and a magnetic head and a magnetic memory device both of which using thereof.
2. Description of the Related Art
Magnetoresistance effect is a phenomenon that the electric resistance of a certain kind of magnetic material varies through application of magnetic field. A magnetoresistance effect element (MR element) taking advantage of magnetoresistance effect is in use in a magnetic head, a magnetic sensor and the like. Further, a magnetoresistance effect memory or the like using a magnetoresistance effect element is proposed. In such an MR element, high sensitivity to an external magnetic field, quicker response and the like are required.
An MR element using a ferromagnetic material is excellent in its thermal stability and an applicable temperature range thereof is large. For an MR element using a ferromagnetic material, a thin film of a ferromagnetic alloy such as a NiFe alloy has been used. Through application of such an MR element for a read head of a hard disc and so on, high density magnetic recording is attained. However, an MR element using a thin film of a NiFe alloy has, since magnetoresistance change rate(MR change rate)is such small as about 2 to 3%, a problem that, when tried to attain further higher density recording, sufficient sensitivity can not be obtained.
Besides, recently, as a new material displaying magnetoresistance effect, an artificial lattice film stacked alternately a ferromagnetic layer and a non-magnetic metal layer with a period of a few nanometers is attracting attention as a material displaying a giant magnetoresistance effect. For instance, a magnetoresistance effect material in which magnetic moments of ferromagnetic layers disposed oppositely through a non-magnetic layer such as an artificial lattice film of Fe/Cr(Phys. Rev. Lett. 61, 2472(1988)) or an artificial lattice film of Co/Cu(J. Mag. Mag. Mater. 94, L1(1991), Phys. Rev. Lett. 66, 2152(1991)) are magnetically coupled in an anti-parallel state is found.
The above described artificial lattice films display a magnetoresistance effect change rate of several tens % remarkably larger than that of a conventional permalloy alloy thin film. Such a giant magnetoresistance effect (GMR) is caused through electron scattering dependent on a direction of the spin of a ferromagnetic layer. However, an artificial lattice film has a problem that numbers of stacked layer has to be increased to obtain an enough large magnetoresistance effect or a problem that, since saturation magnetic field (magnetic field where the resistance value saturates) is such large as several teslas (T) or more, application to a magnetic head and the like by itself is not suitable.
There, with an objective to reduce a saturation magnetic field, a spin valve film having a laminate film of a sandwich structure of ferromagnetic layer/non-magnetic layer/ferromagnetic layer has been developed. In a spin valve film, exchange bias is affected to one ferromagnetic layer to pin its magnetization, the other ferromagnetic layer is reversed in its magnetization through external magnetic field. Thus, the relative angle of magnetization directions of 2 ferromagnetic layers is varied to obtain magnetoresistance effect. However, a spin valve film is not sufficient in its MR change rate and resistance itself of a laminate film is such small as several tens .mu..OMEGA.cm. Therefore, there is a problem that relatively large electric current is required to detect an external magnetic field.
In addition, a very large magnetoresistance effect is known to be obtained when utilizing a so-called perpendicular magnetoresistance effect in which an electric current is flowed to a magnetic multi-layer film in a direction perpendicular to a film plane (Phys. Rev. Lett. 66, 3060(1991)). However, in this case, there is a problem that, since the path of electric current is short and resistance is small due to each layer being metal, without finely processing down to sub-micron or less, magnetoresistance effect at room temperature can not be measured.
Different from the above described multi-layer structures, a so-called granular magnetic film dispersed magnetic ultra-fine particles in a non-magnetic metal matrix is also known to show a giant magnetoresistance effect based on spin-dependent conduction (Phys. Rev. Lett. 68, 3745(1992)). A granular magnetic film is high in its resistance when magnetic field is not applied because the spin of each magnetic ultra-fine particle is randomly aligned each other, and the resistance decreases when each spin is aligned to a direction of magnetic field through application of magnetic field. As a result, a magnetoresistance effect based on spin-dependent scattering appears. Since manufacturing of granular magnetic film is easy compared with such as an artificial lattice film, it is expected as a magnetoresistance effect element of next generation. However, since, in a conventional granular magnetic film, magnetic ultra-fine particles display superparamagnetism, there is a problem that saturation magnetic field is intrinsically very large.
A giant magnetoresistance effect different in mechanism from the above described spin-dependent scattering and based on ferromagnetic tunnel effect is also found. This is to generate a tunnel current through application of electric voltage between both ferromagnetic layers in a structure which is composed of a laminate film of 3 layers of ferromagnetic layer/insulating layer/ferromagnetic layer and in which coercive force of one ferromagnetic layer is small than that of the other ferromagnetic layer. In this time, if only the spin of the ferromagnetic layer small in its coercive force is reversed, since a tunnel electric current is largely different depending on whether spins of 2 ferromagnetic layers are parallel or anti-parallel, a giant magnetoresistance effect can be obtained.
Such a ferromagnetic tunnel junction element has a feature that its structure is simple and, at room temperature, magnetoresistance change rate such large as about 20% can be obtained. However, in order to make the tunnel effect appear, such thin thickness of an insulating layer as less than several nanometers is required. Since stable and homogeneous manufacturing of such a thin insulating layer is difficult, the magnetoresistance change rate tends to vary largely. When the resistance of an insulating layer is too high, in case of using this for a memory element, such problems are expected that the quick response of the element can not be obtained and noise is increased to reduce S/N ratio. Further, if increased an electric current which is flowed to a ferromagnetic tunnel junction element to obtain a desired output voltage, the magnetoresistance change rate drastically decreases to result in a problem(Phys. Rev. Lett. 74, 3273(1995)).
As described above, a granular magnetic film dispersed magnetic ultra-fine particles in a matrix is easy in its manufacturing compared with an artificial lattice film and the magnetoresistance change rate thereof such large as about 10% can be obtained at room temperature. Further, since the ultra-fine particles are small in their particle diameters such as several tens angstroms or less enough to be mono magnetic domain, hysteresis of a magnetoresistance curve is small. Therefore, when used as an MR element, Barkhausen noise is expected to be small. However, because of superparamagnetism of conventional ultra-fine particles, saturation magnetization is intrinsically very large, resulting in a problem when being put into practical use.
Besides, a ferromagnetic tunnel junction element shows a magnetoresistance change rate such large as about 20% at room temperature and a small saturation magnetic field, but there is a problem that manufacturing of an element having stable properties is difficult due to a very thin film of an insulating layer. In addition, when an electric current which is flowed to a ferromagnetic tunnel junction element is increased to obtain a desired output voltage value, a problem appears that the MR change rate decreases drastically.